In many applications of single-mode optical waveguides, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. The rotation of the polarization plane of a propagating signal can be prevented or reduced by employing a fiber that is birefringent, i.e. the refractive index of the core thereof is different for two orthogonally polarized light waves.
The polarization performance of a single-mode fiber can be characterized by its beat length L, where L is defined as 2.pi./.DELTA..beta., and .DELTA..beta. is the difference in the propagation constants of the two polarizations.
The inventions disclosed in U.S. Pat. Nos. 4,179,189 and 4,274,854 are based upon the recognition that orthogonally polarized waves are more efficiently decoupled in a waveguide that is fabricated in such a manner as to deliberately enhance stress-induced, or strain birefringence. Those patents teach that such behavior is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. The strain-induced birefringence is introduced by at least partially surrounding the single mode waveguide by an outer jacket having a different thermal coefficient of expansion (TCE) than that of the waveguide and a thickness along one direction that is different from its thickness along a direction orthogonal to the one direction. This type of fiber has been formed by first depositing within a silica tube a layer of inner cladding material and then a layer core material. Either before or after collapsing the resultant tubular preform to eliminate the central hole, diametrically opposed portions of the outer surface of the tube are ground flat. During the fiber drawing operation, the outer cladding of the drawn fiber will assume a round cross-section, whereas the inner cladding becomes elliptical. The minimum thickness of the elliptically-shaped inner cladding, when formed in this manner, is insufficient to obtain the desired stress on the core.
The aspect ratio of the inner cladding can be increased somewhat by employing the method of U.S. Pat. No. 4,360,371 wherein a tubular intermediate product may be formed by a chemical vapor deposition (CVD) technique whereby one or more layers are deposited on the inner surface of substrate tube. The innermost layer forms the core and at least one of the other layers, which is thicker than the core layer, forms the inner cladding. In another embodiment thereof, the tubular intermediate product is formed by a flame oxidation technique. Reactant vapors are fed to a burner where they are oxidized in a flame to form glass soot which is deposited on a cylindrical mandrel. The first applied soot layer forms the core material of the resultant fiber. At least one additional layer of soot is applied to the first layer to form the inner cladding. After the mandrel is removed, the resultant hollow soot preform is consolidated to form a tubular intermediate product. The intermediate product formed by either of these processes is collapsed to a flattened preform foreproduct wherein the core glass has been transformed into a unitary layer having an elongated cross-section. This core layer is surrounded by an inner cladding layer which now has an oblong cross-sectional configuration. A layer of flame hydrolysis-produced soot is deposited on the outer surface of the inner cladding layer, the TCE of the soot being different from that of the inner cladding glass. The resultant article is heated to consolidate the soot into an outer cladding glass layer, thereby forming a solid glass draw blank which can be drawn into an optical waveguide fiber.
Some disadvantages of the aforementioned prior art are as follows. When a substrate tube forms a part of an optical fiber near the core, fiber attenuation can be adversely affected because of the amount of impurities contained in such a substrate tube as compared with glass that is vapor deposited. Also, most prior art processes result in an oblong core, a feature which can render the splicing of fibers more difficult. Furthermore, it is difficult to employ these prior art methods to form fibers in which minimum inner cladding thickness is extremely small. As the minimum inner cladding thickness approaches zero, the stress which can be applied to the core increases, thereby increasing the polarization retaining capacity of the fiber.